"As you may or may not know, Windows 7 allows two display drivers to be used at once – like in Windows XP. Therefore, it is possible to use an Nvidia card for PhysX and ATI card for graphics rendering. Sadly, since the release of 186 graphics drivers, Nvidia has decided to block this feature anytime a Non-Nvidia GPU is present in the system. In addition, for some incomprehensible reasons, the latest version of PhysX System Software also prevents PPU cards from working if a Non-Nvidia GPU is present.
As expected, this move by Nvidia generated a lot of criticism from both consumers and even Nvidia’s competitors. Luckily, a forum member by the name of GenL has released an experimental beta patch which intercepts disable-PhysX-if-Radeon-is-present-code. So far, according to user comments the patch delivers successful results. At this stage, the patch only works for PhysX GPU rendering and not PPU rendering. However, the developer claims that work is still in progress."